1. Technical Field
The present invention relates to scanned-slit x-ray imaging in mammography wherein for the first time a high spatial resolution image can be registered without compromising the agronomy in positioning the breast. It further relates to a beam collimator arrangement consisting of one or several collimators in an x-ray apparatus shaping the x-ray beam to hit the active detector area (pre-collimator) and to remove scattered x-rays (aft collimator).
2. Background Information
When carrying out scanned-slit mammography, one critical problem is placing the breast of a patient in the correct position while at the same time also positioning the moving collimator as close as possible to the breast in order to obtain a high quality image with respect to spatial resolution.
FIG. 1a provides a schematic side view of a digital mammography x-ray equipment 10 having an x-ray source 11, a collimator arrangement 12, compression paddles 13 and 14, and a detector assembly 15. X-rays are denoted with 17. The breast 16 to be examined (x-rayed) is placed between the compression paddles 13 and 14 and exposed to pressure. This pressure prevents motion during the exposure, and also decreases the amount of overlapping tissue since the breast is flattened out.
As illustrated in FIGS. 1a and 1b, the collimator arrangement 12 must be placed as close as possible to the upper compression paddle 12. As indicated in FIG. 1a, the image quality is compromised if this does not happen. This is because the size of the x-ray source imaged on the detector through the slit the spatial resolution b1 will deteriorate if the collimator is moved away from the object. This is only true is one dimension—the dimension orthogonal to the slit as indicated in FIG. 1b. Thus, when placing the breast in the examining position one must avoid the collimator arrangement, and when the examining starts the collimator must be placed as close as possible to the breast above the upper paddle.
European Patent No. 0417965 discloses a method and apparatus for improving imaging in an X-ray mammography machine having a controllable X-ray source. The invention incorporates apparatus for directing the X-ray beam in a preselected configuration onto at least a portion of a target area. The intensity of the beam created is detected in each of a plurality of substantially equal segments of the target area and a first segment of lowest radiation intensity is identified. The X-ray source is adjusted to establish a predetermined radiation intensity in the identified first segment. Second segments are then identified each having a radiation intensity greater than a predetermined percentage of the radiation intensity in the identified first segment. The radiation intensity directed toward the identified second segments is then automatically attenuated by a control system responsive to the radiation intensity detected in the second segments. The attenuation is achieved by moving collimators into the beam so as to reduce the radiation directed towards the second segments. The beam is scanned over the target and continuously adjusted in accordance with the radiation detected in each of the segments of the beam. In this manner, each area of the target receives just sufficient radiation to provide proper imaging without overexposing area of the target, which have less attenuation than other areas. Thus, the object of this invention is to obtain attenuation by horizontally displacing the collimator.
A mammography apparatus for obtaining mammography images with optimal beam collimation is disclosed in U.S. Pat. No. 5,627,869. Proportional control of collimator element position based upon compression paddle position and dimension inputs is provided to obtain optimal collimation of the X-ray beam for a desired imaging technique and compressed breast thickness. Relevant objects of this invention include providing a mammography apparatus with an X-ray beam collimator that is automatically adjusted to provide an appropriately collimated X-ray beam for any magnification factor, and to provide a mammography apparatus with an X-ray beam collimator that is automatically adjusted, thereby providing an X-ray beam that is appropriately collimated for any magnification factor for a breast of any thickness using a compression paddle of any dimension. As such, the collimator is provided with collimator blades that are controlled so as to be displaced in horizontal direction.
U.S. Pat. No. 4,203,037 discloses an apparatus for exposing an object to x-ray radiation. The apparatus includes an adjustable object supporting and compressing means, and a first collimator means having an opening and that is disposed between the x-ray generator and the object supporting means. A second collimator means having an opening and that is disposed between the object supporting means and x-ray detecting means is also provided. Means are provided for moving the first and second collimator means so that the alignment between the openings therein and the x-ray generator is maintained. This is done in order to expose portions of the object passing through the first collimator means in a sequential fashion while being x-rayed, and permit the x-ray to pass through the opening in the second collimator means and impinge upon the x-ray detecting means. The object supporting and compressing means is preferably structurally independent of the collimator means. The x-ray detecting means preferably takes the form of a highly sensitive detector such as a film-screen cassette or a self-scanning array of photodiodes optically coupled to a scintillator means.